In order to ensure the interoperability among object representations and even between different object systems that uses the same representation. In this context we need to define the HOW and REPR API, this is the SMOP OO API that allows representations to be interchanged, and custom BUILD methods to be written. The interesting aspect is that all this methods must be called in the object itself in SMOP, and that's why they have the "^" and "^!" prefixes, where the first mean a HOW call, and the second means a REPR call.

It is important to stress that the REPR API is SMOP specific, whereas the HOW API is supposed to be transversal to Perl 6 implementations.

Attribute API

Method API

method call($method: $capture --> Object)

HOW API

This is the instrospection API, that is available as standard Perl 6 and relate to the Object considering all the inheritance and composition. This represent all object capabilities, including the inherited ones, not only the ones implemented by this specific instance (as opposed to the REPR API).

The HOW calls are made using the HOW object as the responder interface, but still giving the object as the invocant. The name in that calls still keep the "^" prefix. This means that the HOW is the association of a given implementation to a given object, which means that the HOW method dispatch won't look inside the invocant for its method table, but will keep it somewhere else, meaning that one metaclass implementation will work exactly the same independent of the object.

REPR API

The representation API defines how the runtime can ask different representations. This API is the real enabler on having different representations, even for the same classes. Unlike the HOW API, this only applies to this specific object. The responder interface is supposed to intercept that calls and never call them using standard MRO. None of the information returned by the methods of the REPR API reflects inherited behaviours, it reflects only the definitions made in that object.

Defined/Undefined

The default "defined" implemented by Object, delegates the call to the representation. By default, an object is defined if there is any instance storage initialized. Which basically means that an object is undefined after CREATE and will become defined after BUILDALL.

method ^!defined($object: --> bool ) {...}

Skipping lookup on instances

The instance method defines if this object is a direct instance of some class. This means that it might have only a single isa, no role and it must not have any other definition. If this value is true, then no lookup is made directly in this object, which means the return for WHAT will be the isa of this object. On the other hand, if this returns false, this is a prototype.

In order to avoid defining Undef, you can only add instance storage to "instance" values. You can't have a "defined" class.

The HOW for instances is the HOW of the isa, non-instance values must define their own HOW which usually is copied when you do Object.clone() to create a new instance.

method ^!instance($object: --> bool ) {...}

RW introspection API

Trying to access the private storage of a class (which returns false to "instance") will result in a failure. Trying to add definitions to a defined object will result in a failure.